This invention relates to a frequency synthesizer and, more particularly, to a synthesizer employing a comb or harmonic frequency generator with comb frequency line selection and a phase locked loop (PLL) for filtering out spurious signal components, the synthesizer having a single side band mixer within the PLL for offsetting the output frequency of the synthesizer from a comb frequency line.
Synthesizers of high frequencies, in the gigahertz range by way of example, frequently employ a phase locked loop to stabilize the output signal frequency, and include also circuitry for down conversion in frequency of a sample of the output signal to enable use of lower frequency circuit components, such as those operating in the megahertz range, for phase locking the output signal to an input reference signal. Generally, it has been the practice to employ a frequency divider comprising one or more digital counters to accomplish the down conversion of frequency.
In the operation of the frequency divider, the counters divide both the frequency and the phase of the sample of the output signal. This operation is satisfactory for many applications, but suffers from a limitation in that there may be excessive phase noise in-the output signal for applications requiring a precision output signal. For example, in the case of a divider which divides the frequency and the phase by a factor of ten for presentation to a phase detector of the phase locked loop, a phase deviation at the low frequency end of the divider is one tenth that at the high frequency end of the divider. Correspondingly, the phase noise power at the output signal is greater than the phase noise power at the input to the phase detector by a factor of 100 (20 decibels).
It is desirable to employ a harmonic or comb frequency generator to obtain the desired output frequency of the synthesizer because the output frequency is then known accurately as an integral multiple of an input reference frequency. However, the use of the aforementioned down conversion with a comb spectrum may introduce aliasing with consequent difficulty in extraction of the desired signal frequency. Furthermore, it is generally necessary to provide the capability to offset an output frequency of the synthesizer from the frequency values of the comb frequency lines.
Thus, there are problems in the construction of a frequency synthesizer employing a comb frequency generator and a frequency divider. In the case of the comb frequency generator, there may be the aforementioned difficulty in the down conversion and in the offsetting of output frequency from a comb line. Furthermore, use of a frequency divider in a phase locked loop introduces a phase noise in the output signal which may be excessive for generation of precision frequency in the output signal. Filtering of spurious signals within a sequence of counters of a divider has not been possible, generally, because of the need for constructing the divider with wide bandwidth to accommodate a wide range of output frequencies from the synthesizer.